Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide generally in the presence of a sulfuric acid catalyst into equimolar amounts of phenol and acetone.
It is also known that phenol and cyclohexanone can be co-produced by a process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, in which the cyclohexylbenzene is produced by hydroalkylating benzene in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide and then decomposed to the desired phenol and cyclohexanone co-product.
Although the process of the '513 patent is very selective in the conversion of benzene to cyclohexylbenzene, one inevitable by-product of the process is cyclohexane resulting from competing hydrogenation of the benzene feed and the cyclohexene intermediate. Not only does the cyclohexane by-product represent a significant loss of valuable benzene feed but its separation from the unreacted benzene by distillation is very difficult since the difference in boiling point between benzene and cyclohexane is only about 1° C. To obviate these problems, it has been proposed to selectively dehydrogenate the cyclohexane in the C6 fraction of the hydroalkylation effluent to produce additional benzene and hydrogen. The unreacted benzene and the benzene and hydrogen produced by dehydrogenation of the cyclohexane can then be recycled to the hydroalkylation step. Examples of this process are disclosed in U.S. Pat. No. 7,579,511 and WO2009/131769.
Investigation of the cyclohexane dehydrogenation process has, however, now shown that the process produces small quantities, of the order of 1000 ppmw, of toluene which, if not removed, would be transported back to the hydroalkylation reactor in the benzene and hydrogen recycle streams. While the exact mechanism is unknown, it is believed the toluene may be produced through the decomposition of cyclohexylbenzene to form toluene and cyclopentane, or a methyl group from methylcyclopentane may react with benzene to form toluene. Toluene is a particularly disadvantageous impurity in the process since, following oxidation and cleavage, it leads to the production of cresols, which are difficult to remove from phenol and represent deleterious contaminants in the phenol product. Thus, the commercial application of dehydrogenation to remove cyclohexane from cyclohexylbenzene will also require removal of the co-produced toluene from the benzene and hydrogen product streams.
However, toluene is a known impurity in commercially available benzene streams, normally at a level of about 100 ppmw. Hence, in practice, any benzene feed used for production of cyclohexylbenzene by hydroalkylation will have to undergo prior treatment, normally by super-fractionation, to reduce the toluene level. Thus, in accordance with the present invention, the benzene recycle stream from the cyclohexane dehydrogenation step is fractionated to remove co-produced toluene and the resultant purified benzene is then used to wash, and thereby remove toluene entrained in the hydrogen recycle stream from the cyclohexane dehydrogenation step. The benzene wash liquid can then be fractionated to remove the toluene transferred from the hydrogen recycle stream. In general, a single super-fractionation column will be used to remove toluene from the feed benzene, the recycle benzene from the cyclohexane dehydrogenation step, and the benzene used to wash the hydrogen recycle from the cyclohexane dehydrogenation step.